Which colors of leads indicate a 120-V AC primary on a step-down transformer?

The choice indicating that black and white leads signify a 120-V AC primary on a step-down transformer is foundational in understanding electrical wiring and transformer color codes. In many electrical systems, particularly in HVAC applications, black typically denotes a hot wire, which carries the voltage. White, on the other hand, is conventionally used for the neutral wire.

In the context of a step-down transformer, which is designed to reduce voltage levels for various applications, the identification of these colors helps technicians safely and accurately connect the primary side of the transformer to a 120-V AC power source. Knowing that black represents the supply voltage and white represents the return path ensures proper functioning and safety in the electrical circuit.

The other options represent different configurations or color codes not typically associated with a standard 120-V AC primary in a transformer setup. Therefore, the identification of tape colors is crucial for maintaining safety and efficiency in HVAC electrical installations.